The present invention relates to an etching process for the purpose of removing the binder phase from the surface of hard material inserts before applying coatings on said surface.
Coated cemented carbide inserts have for many years been commercially available for the chip cutting machining of metals in the metal cutting industry. Such inserts are commonly made of a metal carbide, normally WC, generally with the addition of carbides of other metals such as Nb, Ti, Ta, etc., and a metallic binder phase, generally of Co. By depositing a thin layer of a wear resistant material such as TiC, TiN, Al.sub.2 O.sub.3, etc., separately or in combination onto said cemented carbide inserts it has been possible to increase the wear resistance without adversely affecting the toughness. A still further improvement in properties has been achievable by subjecting the inserts to a binder phase enrichment in the surface below the coating, a so-called cobalt gradient. Binder phase enrichment can be accomplished, for instance, by sintering in vacuum while adding nitride as is disclosed in U.S. Pat. No. 4,610,931 or by controlled cooling as discussed in U.S. Pat. No. 4,911,989. Such inserts, however, often also appear with a thin layer of binder phase on their surface and sometimes they even appear with a layer of graphite thereon. The latter two types of layers have a negative effect on the process when carrying out CVD- or PVD-deposition which results in layers with inferior properties and insufficient adherence. These layers must therefore be removed before carrying out the deposition process.
It is possible to remove such cobalt and possible graphite-layers mechanically by blasting. The blasting method is, however, difficult to control. The difficulty resides in the inability to consistently control the blasting depth with necessary accuracy which leads to an increased scatter in the properties in the final product--the coated insert.
Chemical or electrolytic methods could be used as alternatives for mechanical methods. U.S. Pat. No. 4,282,289 discloses a method of etching in a gaseous phase by using HCl in an initial phase of the coating process. In U.S. Pat. No. 4,911,989 there is disclosed a wet chemical method of etching in nitric acid, hydrochloric acid, hydrofluoric acid, sulfuric acid or similar or electro-chemical methods. From JP 88-060279 it is previously known to use an alkaline solution, e.g., NaOH, and from JP 88-060280 to use an acid solution. JP 88-053269 discloses etching in nitric acid before diamond deposition. There is one drawback with these methods, namely, that they are incapable of only removing the cobalt layer. They also result in deep penetration particularly in areas close to the edge. The etching medium not only removes cobalt from the surface but also penetrates areas between the hard material grains, the result of which is an undesired porosity between the layer and the substrate at the same time as the cobalt layer may partially remain in other areas of the insert.